1. Technical Field
The present invention relates generally to uplink scheduling in a wireless communication network using interference suppression receivers at the base station, and more particularly to load prediction for interference suppression receivers.
2. Background
In some types of wireless communication systems, such as Wideband Code Division Multiple Access (WCDMA) systems, interference suppression (IS) is used in order to achieve better performance in terms of peak data rates, coverage, system throughput, and system capacity. Examples of such interference suppression receivers that are commonly used include the GRAKE+ receiver, the Frequency Domain Equalization (FDE) receiver, and the Frequency Domain Pre-Equalization (FDPE) receiver.
The Enhanced Uplink (EUL) in WCDMA employs fast scheduling and a short transmission time interval (TTI) in order to use increase total throughput on the uplink and reduce delay. For EUL, the users report channel conditions and buffer status to the base station. A scheduler at the base station determines which users are allowed to transmit in a given TTI and at what data rate based on the channel and load conditions. Because the transmissions on the uplink are not orthogonal, the uplink transmissions by each user creates interference for the other users. If the interference level is too high, the base station may not be able to receive some transmissions. On the other hand, if the interference level is too low, the full system capacity will not be exploited.
In order to maintain the interference at acceptable levels, the scheduler at the base station estimates the load at the base station due to its scheduling decisions and tries to maintain the total load below a predetermined threshold. Prediction of the uplink load is difficult because of the inherent delay between the time that scheduling decisions are made and the time that the users transmit on the uplink. In conventional systems that do not employ interference suppression receivers, an estimate of the predicted load is based on measurements of the signal-to-interference ratio (SIR) together with beta factors that define the power offset of a user relative to a power-controlled control channel. This technique is suitable for conventional receivers that do not employ interference suppression.
The introduction of interference suppression receivers in the near future will make the load prediction problem substantially more difficult for several reasons. First, the load appearing over the air interface will be significantly reduced by the interference suppression receivers. As a result, the conventional load measures based on measurements of the SIR will not reflect the load after interference suppression that is applicable to interference suppression receivers. Second, the amount of the load reduction due to interference suppression receivers will vary depending on the number of interfering users in the cell. Third, the amount of the load reduction will vary depending on the data rates of the users transmitting on the uplink.
Accordingly, there remains a need for techniques to predict an “effective load” after interference suppression that is applicable to interference suppression receivers.